docs: def _prepare_body(self):
          """ private function to prepare content for paramType=body
          """
          content_type = self.__consume
          if not content_type:
              content_type = self.__op.consumes[0] if self.__op.consumes else 'application/json'
          if self.__op.consumes and content_type not in self.__op.consumes:
              raise errs.SchemaError('content type {0} does not present in {1}'.format(content_type, self.__op.consumes))
  
          # according to spec, payload should be one and only,
          # so we just return the first value in dict.
          for parameter in self.__op.parameters:
              parameter = final(parameter)
              if getattr(parameter, 'in') == 'body':
                  schema = deref(parameter.schema)
                  _type = schema.type
                  _format = schema.format
                  name = schema.name
                  body = self.__p['body'][parameter.name]
                  return content_type, self.__op._mime_codec.marshal(content_type, body, _type=_type, _format=_format, name=name)
          return None, None